Ti-Nb-Zr Alloy

ABSTRACT

An object of the present invention is to develop a shape memory and superelastic alloy that does not contain nickel, and has superelasticity and shape memory properties even if being subjected to heat treatment in spite of high biocompatibility, moreover having high cold workability. The Ti—Nb—Zr base alloy is comprising an alloy composition consisting of 40 to 60 wt % Ti, 18 to 30 wt % Nb, 18 to 30 wt % Zr, and 0.77 to 3.7 wt % at least one metal additional element selected from Al, Sn, In and Ga. The Ti—Nb—Zr base alloy is a practical alloy in which the principal components form a strong and dense oxidation film to exhibit high biocompatibility, and also the alloy has superelasticity and shape memory properties, high cold workability, and high low-temperature properties.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a shape memory and superelastic alloy having high cold workability and moreover having high biocompatibility.

2. Background Art

Shape memory properties mean a phenomenon that although a large residual strain occurs when a load is applied to a metal having a certain particular shape (for example, a coil shape) given at the start to plastically deform the metal by elongating it exceeding the elastic limit and then the load is removed, the metal returns into the original shape (original coil shape) when it is heated, namely, a phenomenon utilizing what is called martensitic transformation—reverse martensitic transformation. The shape memory properties have so far been utilized for turbine-type thermal engines, pipe joints, and brassiere wires, for example.

On the other hand, superelasticity means a phenomenon that when a load is applied to a metal in the same way as described above to plastically deform the metal by elongating it exceeding the elastic limit and the load is removed from this state, the metal returns into the original state (a state in which the residual strain is almost zero) while drawing displacement trace of a hysteresis. In this phenomenon, martensitic transformation—reverse martensitic transformation develops without raising the temperature. This phenomenon is basically the same as the shape memory properties. In other words, in the case of shape memory properties, heat is added to eliminate residual strain, whereas, in the case of superelasticity, heat need not be added. As applications in which superelasticity is used, a medical guide wire and the like are cited.

As a shape memory and superelastic alloy, a Ti—Ni alloy is typically cited. The Ti—Ni alloy has a unique property that it can be formed into a plate or a wire by plastic working although it is an intermetallic compound, and also it undergoes martensitic transformation—reverse martensitic transformation at both ends of a particular temperature range near room temperature, thereby achieving the shape memory properties and superelasticity.

However, since the Ti—Ni alloy contains Ni as a component, the biocompatibility thereof is poor, so that in particular, it cannot be used for a member relating to the human body. In addition, the Ti—Ni alloy undergoes martensitic transformation—reverse martensitic transformation at a temperature near room temperature, so that it also has a problem in that when the temperature lowers to −50° C. or lower, it softens suddenly and thus the low-temperature properties thereof are degraded. In such circumstances, there has been attempted the development of a shape memory and superelastic alloy that does not contain Ni toxic to the human body and is superior in a low temperature properties; see Ti—Sc—Mo KEIJO KIOKU GOKIN NO KAIHATSU, Nihon-Kinzoku gakkai, syunkidaikai koengaiyo,144(45) [Development of Ti—Sc—Mo Shape Memory Alloy, The Japan Institute of Metals, 2003 Spring Time Annual Proceedings, page 144(45)].

However, all of the alloys having been developed so far have problems in that the elongation is low, the superelasticity and shape memory effect is poor, and the availableness as a practical alloy is poor. Besides, all of the Ni-free shape memory alloys have a drawback in that because of poor cold workability, they are difficult to use as an industrial material.

The present invention has been made in view of the above problems with the conventional alloys, and an object thereof is to develop a shape memory and superelastic alloy that does not contain nickel, and has superelasticity and shape memory properties in spite of high biocompatibility, moreover having high cold workability.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, a Ti—Nb—Zr base shape memory alloy in accordance with the present invention, and is characterized by “an alloy composition consisting by weight of 40 to 60 percent Ti, 18 to 30 percent Nb, 18 to 30 percent Zr, and 0.77 to 3.7 percent at least one metal additional element selected from Al, Sn, In and Ga.”

The principal elements of this alloy are Ti, Nb and Zr that form a strong and dense oxidation film to provide biocompatibility, so that this alloy can be used as a biomaterial. Moreover, this alloy is a practical material that has high cold workability such as to withstand cold working of 50% or more (if working is divided into several cycles, 95% or more without annealing) by one cycle. Also, since martensitic transformation—reverse martensitic transformation occurs in the temperature range of −50° C. to −20 or −30° C., a sudden decrease in rigidity does not occur at a temperature near 0° C., so that this alloy has low-temperature properties in the practical range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a stress-elongation curve at the time when the alloy in accordance with the present invention contains 1.6 wt % Al;

FIG. 2 is a stress-elongation curve at the time when the alloy in accordance with the present invention contains 1.9 wt % Al;

FIG. 3 is a stress-elongation curve at the time when the alloy in accordance with the present invention contains 2.3 wt % Al;

FIG. 4 is a stress-elongation curve for example 2 of an alloy in accordance with the present invention;

FIG. 5 is a stress-elongation curve for example 3 of an alloy in accordance with the present invention;

FIG. 6 is a stress-elongation curve for example 4 of an alloy in accordance with the present invention;

FIG. 7 is a stress-elongation curve for example 5 of an alloy in accordance with the present invention; and

FIG. 8 is a stress-elongation curve for example 6 of an alloy in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a shape memory alloy containing Ti(Titanium), Nb(Niobium) and Zr(Zirconium) as principal components. Specifically, the alloy has an alloy composition of 40 to 60 wt % Ti, 18 to 30 wt % Nb, 18 to 30 wt % Zr, and 0.77 to 3.7 wt % total of at least one metal additional elements selected from Al, Sn, In and Ga. In the case where two or more metal additional elements are contained, the total amount is 0.77 to 3.7 wt %.

It is known that, of the constituent elements forming the above-described alloy, Ti has a hexagonal close-packed lattice structure (α phase) at room temperature, so that high ductility cannot be anticipated; however, the structure changes to a body-centered cubic lattice structure (β phase) at a temperature of 882° C. or higher, and a ductility higher than that of the α phase appears.

The Young's modulus of pure Ti (grade 2) is 106 GPa, and a dense oxidation film of TiO₂ is formed on the surface at a temperature of 610° C. or higher. This TiO₂ film does not change in the air at room temperature, and has high strength and corrosion resistance. Ti has a high specific strength (quotient of tensile strength divided by specific gravity), so that by utilizing this property, it is often used for an alloy using Ti as the base.

The Ti base alloy is turned into a solid solution by alloying, and exerts a great influence on the ductility. If the ductility is lost, forging that is an important means for improving at least the casting structure cannot be performed. Therefore, the requirement for an excellent Ti base alloy is to have plasticity.

If the content of Ti is too high, the poorness of plasticity and workability, which is an inherent drawback of Ti, appears remarkably, and hence working at room temperature cannot be performed unpreferably. Inversely, if the content of Ti is too low, high strength, specific strength, corrosion resistance, and stability, which are advantages of the use of Ti, are insufficient, so that the excellent properties of Ti do not appear. Therefore, in the alloy in accordance with the present invention, 40 to 60 wt % Ti is needed to satisfy these requirements.

Zr has a hexagonal close-packed lattice structure (a phase) at room temperature, and changes so as to have a body-centered cubic lattice structure (β phase) at a temperature of 862° C. or higher. Also, Zr has a property that a dense oxidation film is formed in the air, the corrosion resistance thereof is high, the corrosion resistance thereof especially in high temperature water is remarkably high as compared with other metals, and Zr is less liable to react even in fused alkali. Since Zr has high corrosion resistance and acid resistance as described above, it is used as an alloy component of a member requiring biocompatibility or in various machinery applications.

If the content of Zr is too low (less than 18 wt % in the present invention), the plasticity and workability thereof decrease remarkably, the Young's modulus thereof increases, and the compatibility with biomedical tissue also becomes low unpreferably. In contrast, if the content of Zr is too high (more than 30% wt % in the present invention), the improvement in corrosion resistance is not found and the specific gravity of the obtained alloy merely increases, the Young's modulus thereof likewise increases, and the plasticity and workability thereof decrease unpreferably. For this reason, by setting the content of Zr in a particular range as described in claims, Zr forms a dense oxidation film without changing Ti in the air at room temperature, so that high corrosion resistance and acid resistance are exhibited by the synergism of characteristics of both elements.

Nb exhibits high malleability, the Young's modulus thereof is 105 GPa, and the hardness thereof is at the same level as that of wrought iron. The addition of Nb gives suppleness (low elasticity) to the obtained alloy. Also, Nb is a metal that forms an oxidation film in the air and thus exhibits corrosion resistance. Therefore, by making Nb a constituent element of the Ti—Zr base alloy of the present invention, the corrosion resistance and acid resistance can be improved in cooperation with Zr.

In the present invention, the content of Nb is in the range of 18 to 30 wt % with respect to the total weight of constituent elements. If the content of Nb is lower than 18 wt %, there arise problems in that the suppleness of the obtained alloy is insufficient, the plasticity thereof decreases, and the Young's modulus thereof increases. Inversely, if Nb is added exceeding 30 wt %, not only the improvement in suppleness cannot be desired, but also the improvement in corrosion resistance is not found. In addition, the specific gravity merely increases, and the compatibility with biomedical tissue is also not improved. This is likewise unpreferable.

In the above-described Ti—Nb—Zr alloy composition, in the case where the content of at least one metal additional element selected from Al, Sn, In and Ga is 0.77 to 3.7 wt %, the alloy exhibits superelasticity and shape memory properties. If the single or total content of the added metals is lower than 0.77 wt %, there arises a problem in that though the shape memory properties and superelasticity increase, the alloy is too soft as a whole, so that the alloy cannot be used as a biomaterial and for a machinery/equipment part. If the content exceeds 3.7 wt %, the shape memory properties and superelasticity are not exhibited (refer to Table 2), and also the workability decreases.

Table 1 gives the case where the amount of Zr, Nb and Al were changed with Ti being the base. According to this table, in the case where the amount of Zr and the amount of Nb were each smaller than 18 wt % and the amount of Al was 0.77 to 3.77 wt %, and in the case where the amount of Zr and the amount of Nb were each larger than 30 wt % and the amount of Al was 0.77 to 3.77 wt %, the shape memory properties and superelasticity did not appear. Only in the case where the amount of Zr and the amount of Nb were in the range of 18 to 30 wt %, the shape memory properties and superelasticity appeared.

Table 2 gives the superelastic deformation elongation [%] (=elongation—residual strain) in the case where the amount of Zr and Nb were fixed and the amount of Al was changed with Ti being the base. When the amount of Al was 1.3 wt %, the superelastic deformation elongation showed the maximum value, being 4%. The superelastic deformation elongation showed the lower limit value 1.9% or 2.0% when the amount of Al was 0.7 wt % and 3.7 wt %, respectively.

TABLE 1 Presence of shape memory properties wt % Ti Zr Nb Al Shape memory properties BAL 17 17 0.77~3.77 No 17 30 0.77~3.77 No 18 18 0.77~3.77 Yes 18 30 0.77~3.77 Yes 20 20 0.77~3.77 Yes 22 23 0.77~3.77 Yes 23 25 0.77~3.77 Yes 30 18 0.77~3.77 Yes 30 30 0.77~3.77 Yes 31 31 0.77~3.77 No

TABLE 2 Superelastic deformation range wt % Ti Zr Nb Al Sperelastic deformation range % BAL 22 23 0.7 1.9 0.9 2.8 1.1 3.6 1.3 4.0 1.5 3.8 1.7 3.8 1.9 3.7 2.1 3.6 2.3 3.6 2.5 3.0 2.7 2.6 2.9 2.5 3.1 2.4 3.3 2.2 3.5 2.0 3.7 2.0 3.9 1.3

Table 3 gives the case where the amount of Zr and Nb were fixed and one metal additional element selected from Al(1.3 wt %), Sn(3 wt %), In(3.5 wt %) or Ga(1.3 wt %) was added with Ti being the base. In all cases, the shape memory properties and superelasticity appeared.

TABLE 3 Shape memory properties of additional element Shape memory Additional element Added amount properties Zr = 22% Al 1.3 Yes Nb = 23% Sn 3.0 Yes Ti = BAL In 3.5 Yes Ge 1.3 Yes

EXAMPLE 1

Three kinds of plates containing 23 wt % Nb, 22 wt % Zr, and 1.6, 1.9 and 2.3 wt % Al, the balance being Ti alloy were prepared. These plates were subjected to solution heat treatment by heating at 800° C., and then were water quenched. The stress-elongation curves of the three kinds of specimens are shown in FIGS. 1 to 3. In all cases, a plastic deformation elongation of 4% was given to the plates, then the load was removed. In the case of FIG. 1 (Al: 1.6 wt %), a residual strain of about 2% was developed, and as a result, a superelastic deformation elongation of 3.8% was exhibited as given in Table 2. In the case of FIG. 2 (Al: 1.9 wt %), a residual strain of about 3% was developed, and as a result, a superelastic deformation elongation of 3.7% was exhibited as given in Table 2. In the case of FIG. 3 (Al: 2.3 wt %), a residual strain of about 4% was developed, and as a result, a superelastic deformation elongation of 3.6% was exhibited as given in Table 2. It is found that the superelastic elongation is changed by the change in amount of Al. All tests in this example were tensile tests proceeded at room temperature, and were not tests proceeded in a region of martensitic transformation—reverse martensitic transformation occurring in the temperature range of −50° C. to −20 or −30° C., so that only superelastic deformation was exhibited. Needless to say, if the tests were proceeded in the martensitic transformation—reverse martensitic transformation region, the shape memory properties were naturally exhibited (FIGS. 1 to 3).

EXAMPLE 2

The stress-elongation curve in the case where a plate (width=2.47 mm, thickness=0.9979 mm) containing 23.2 wt % Nb, 22.1 wt % Zr, and 1.27 wt % Al, the balance being Ti alloy was subjected to solution heat treatment by heating at 800° C., and then was water quenched is shown by a solid line. When a stress of 520 MPa was applied, a plastic elongation of 4% was exhibited, and when the stress was removed, a residual strain of about 0.2% was developed, and superelasticity (superelastic elongation is about 3.8%) was exhibited (FIG. 4).

EXAMPLE 3

A plate (width=2.47 mm, thickness=1.001 mm) containing 23.2 wt % Nb, 22.1 wt % Zr, and 1.27 wt % Al, the balance being Ti alloy was subjected to solution heat treatment by heating at 800° C. for 30 minutes. The tensile stress (MPa) vs. elongation (%) of this plate was as shown in FIG. 5. When a stress of 699.2 MPa was applied at the first time, an elongation of 4% was exhibited, and when the stress was removed, a residual strain of 0.7% was present. When a stress of about 731.05 MPa was applied again, an elongation of 4.7% was exhibited, and when the stress was removed, a residual strain of 0.72% was developed from the point of load 0 at the first time. The figure reveals that the superelasticity (and shape memory properties) appeared (FIG. 5).

EXAMPLE 4

A plate (width=2.47 mm, thickness=0.9979 mm) containing 23.2 wt % Nb, 22.1 wt % Zr, and 1.27 wt % Sn, the balance being Ti alloy was subjected to solution heat treatment by heating at 800° C. for 30 minutes, and then was water quenched. The tensile stress (MPa) vs. elongation (%) of this plate was as shown in FIG. 6. When a stress of 670.27 MPa was applied at the first time, a plastic elongation of 4% was exhibited, and when the stress was removed, a residual strain of 0.56% was present. When a stress of about 702.39 MPa was applied again, a plastic elongation of 4.4% was exhibited, and when the stress was removed, a residual strain of 0.44% was developed from the point of load 0 at the first time. In this case as well, the figure reveals that the superelasticity (and shape memory properties) appeared (FIG. 6).

EXAMPLE 5

A plate containing 23.2 wt % Nb, 22.1 wt % Zr, and 1.27 wt % In, the balance being Ti alloy was subjected to solution heat treatment by heating at 800° C. for 30 minutes. In this case, when a stress of 520 MPa was applied, a plastic elongation of 4% was exhibited, and when the stress was removed, a residual strain of 0.2% was developed, so that a superelastic elongation of 3.8% (=4%−0.2%) was exhibited. When a stress of 750 MPa was applied, an elongation of 4% was exhibited, and when the stress was removed, a residual strain of 0.9% was developed, so that a superelastic elongation of 3.1% (=4%−0.9%) was exhibited. In this case as well, the figure reveals that the superelasticity (and shape memory properties) appeared (FIG. 7).

EXAMPLE 6

A plate containing 23 wt % Nb, 22 wt % Zr, and 1.3 wt % Ga, the balance being Ti alloy was subjected to solution heat treatment by heating at 800° C. for 30 minutes. In this case, when a stress of 520 MPa was applied, an elongation of 4% was similarly exhibited, and when the stress was removed, a residual strain of 0.05% was developed, so that a superelastic elongation of 3.95% (=4%−0.05%) was exhibited. When a stress of 750 MPa was applied, an elongation of 4% was exhibited. In this case as well, the figure reveals that the superelasticity (and shape memory properties) appeared (FIG. 8).

Also, the alloy having the above-described composition was rolled at a reduction ratio (namely, a reduction in area) of 50%, and no crack was created. If rolling was performed by dividing the rolling operation into several cycles, cold rolling more than 95% was able to be performed without annealing.

According to the present invention, shape memory properties and superelasticity can be given to a Ti—Nb—Zr base alloy, which is inherently a biocompatible alloy having no shape memory properties and superelasticity, by adding at least any one of Al, Sn, In and Ga. Therefore, the Ti—Nb—Zr base alloy in accordance with the present invention can be used especially as a biomaterial (for example, an indwelling stent and a guide wire) in the medical field. By the present invention, new application fields for the Ti—Nb—Zr base alloy can be opened up. 

1. A Ti—Nb—Zr base alloy comprising: an alloy composition consisting by weight of 40 to 60 percent Ti, 18 to 30 percent Nb, 18 to 30 percent Zr; and 0.77 to 3.7 percent at least one metal additional element selected from Al, Sn, In and Ga.
 2. A method for manufacturing a Ti—Nb—Zr base alloy, comprising: subjecting an alloy having an alloy composition consisting by weight of 40 to 60 percent Ti, 18 to 30 percent Nb, 18 to 30 percent Zr, and 0.77 to 3.7 percent at least one metal element selected from Al, Sn, In and Ga to cold working of 50% or more, and subjecting the alloy to solution heat treatment. 